1. Field of the Invention
The present invention relates to an apparatus and method for estimating the internal resistance of a power storage device, an apparatus for determining degradation in the power storage device based on the internal resistance, and a power supply system including a power storage device.
2. Description of the Background Art
Japanese Patent Laying-Open No. 2009-128194 (Patent Document 1), for example, discloses a device to determine degradation in a rechargeable battery. The device includes current detection means for detecting current flowing into a rechargeable battery, ripple factor calculation means for calculating the ripple factor of the detected current, storage means for storing in advance data associating the ripple factor with the internal resistance, and determination means. The ripple factor calculation means divides the current detected by the current detection means into a ripple component and a direct current component, and calculates the ratio of the ripple component to the direct current component as the ripple factor. The determination means calculates the internal resistance of the rechargeable battery based on the data stored in the storage means and the ripple factor calculated by the ripple factor calculation means to determine whether the rechargeable battery is degraded or not based on the internal resistance.
Japanese Patent Laying-Open No. 2007-12568 (Patent Document 2), for example, discloses a control device generating a ripple current at a secondary battery to raise the temperature of the secondary battery. The passage of a ripple current through the internal resistor of the secondary battery causes heat generation in the secondary battery. The temperature of the secondary battery can be increased without using a heater by the heat generation of the secondary battery per se.
The internal resistance of a power storage device generally has temperature dependency. However, the internal resistance of a rechargeable battery is calculated based on only the ripple component, according to Japanese Patent Laying-Open No. 2009-128194. Therefore, the temperature dependency of the internal resistance may affect the determination of degradation in the rechargeable battery according to the method disclosed in Japanese Patent Laying-Open No. 2009-128194. Japanese Patent Laying-Open No. 2007-12568 is silent about estimating the internal resistance of the power storage device, although a method of utilizing the internal resistance of the power storage device is disclosed.